1. Field of the Invention
This invention relates to washing machines including a single-phase induction motor provided for driving a rotational tub in a dehydrating operation and having a main winding and an auxiliary winding, and more particularly to such washing machines wherein the rotational speed of the single-phase induction motor is controlled by exciting selectively either one or both of the main and auxiliary windings.
2. Description of the Prior Art
A single-phase induction motor has been generally used for wash and dehydrating operations in washing machines.It is well known in the art that a desired dehydrating operation can be performed by controlling an ascent curve of the rotational speed of the single-phase induction motor. The prior art has provided for an arrangement that a primary voltage of the single-phase induction motor is controlled by controlling the phase of an AC power supply output. However, a large harmonic voltage inevitably results from the phase control as described above. The generation of such a large harmonic voltage accompanies an electromagnetic noise, which causes a problem of radio interference.
To solve the above-described problem, the prior art has recently proposed an arrangement that the motor speed is controlled by selectively exciting either one or both of the main and auxiliary coils. The harmonic voltage can be reduced in the proposed arrangement as compared with the phase control.
In the proposed arrangement, a developed torque is increased in a control pattern wherein both of the main and auxiliary coils are excited, when the motor speed is in the low or medium speed range or when the slip is relatively large. However, the electromagnetic noise is not abnormally increased since the nonuniformity in the torque is relatively small. On the other hand, where a control pattern wherein only the main coil is excited is selected when the motor speed is in the low or medium speed range, a relatively large torque nonuniformity is caused under the condition that the developed torque is larger than that in the control pattern wherein only the auxiliary coil is excited, resulting in a relatively large electromagnetic noise.
Furthermore, the maximum speed differs between at the power supply frequencies of 50 and 60 Hz when the single-phase induction motor is controlled in the manner as described above with the same load applied thereto. Consequently, the dehydrating performance of the washing machine differs between a region where the power supply frequency of 50 Hz is employed and a region where the power supply frequency of 60 Hz is employed.
To solve the above-described problem, the prior art has proposed the following countermeasure: the power supply frequency is detected and the motor is operated at the normal speed depending upon the detected frequency when it is at 50 Hz. When the detected frequency is at 60 Hz, the final target speed of the motor is controlled to correspond to the normal speed at 50 Hz. However, this countermeasure necessitates the power supply frequency detecting means and control programs need to be changed in the power supply frequencies of 50 and 60 Hz. Consequently, the circuit arrangement is complicated.